Seismic circuit



Aug. 10, 1943. w, H, MAYNE SEISMIC CIRCUIT Filed March l5, 1940 @www Patented Aug. 10, 1943 UNITED SEISMIC CIRCUIT William Harry Mayne, San Antonio, Tex., as-

signor to Olive S. Petty, San Antonio, Tex.

Application March 13, 1940, Serial No. 323,828 l 9 Claims. (Cl. 177-352) This invention relates to novel geophysical prospecting methods and apparatus and more particularly to the firing of the blast which initiates the artificial seismic waves, to the recording of the exact instant of the explosion and to communications between blasting and recording stations.

It is a general object of the present invention to provide novel methods of and apparatus for associating the blasting instrumentalities and the recording apparatus whereby the time-break can be more readily and accurately recorded and terminated so rapidly that its record can be taken on the same trace used for a seismometer.

The improved methods and apparatus include safety means to positively insure against unintentional firing of the blasting cap while yet providing adequate filtration to eliminate high frequency ripples from the time-break trace. It is therefore more specically an object of the invention to interpose a one-way valve means between the bridge wire of the blasting cap and the filter mechanism provided for eliminating the ripple of the blasting machine current from the record which means positively prevents discharge of filter condensers through the cap.

An important feature of the invention comprises the use of a thermionic tube interposed between the blasting circuit and the recording equipment, which is adapted to rectify the high frequency oscillations resulting from the'sudden disruption ofthe bridge wire of the blasting cap to thereby provide a signal that will be recorded as a relatively smooth and brief curve with sharp break points, rather thany a fuzzy unintelligible trace.

The equipment for providing the novel features heretofore referred to is particularly adapted for the provision of telephonie communication between the operator of the blasting equipment and the operator at the recording station, so that the blaster can be apprized of the condition of the equipment and know when to fire the charge.

Another important feature of the invention consists in the use of the signal resulting from voltage changes across the bridge wire of the blasting cap which signal can be appropriately rectified and filtered, to obtain a time-break trace having a sharp break resulting in a superior record even under the most adverse conditions.

The thermicnic tube interposed between the ripple filter and the bridge wire of the blasting cap is provided with a safety feature consisting of a high resistance interposed between the grid of the tube and the bridge wire of the cap to prevent accidental firing of the cap from filter discharges in the event of accidental shorting of the tube elements.

Other and further objects and features of the invention will be more apparent to those skilled in the art upon consideration of the accompanying drawing and following specification wherein is disclosed a single exemplary embodiment of the invention, with the understanding that such changes and modifications may be made therein as fall within the scope of the appended claims without departing from the spirit of the invention.

In said drawing:

Figure 1 is a schematic illustration of the various pieces of equipment necessary to carry out the methods of the present invention, said apparatus being shown connected in an appropriate circuit;

Figure 2 is a schematic wiring diagram of the apparatus contained in the so-called shooters box at the blasting station;

Figure 3 is a wiring diagram of transformer coupling means for use between the blasting circuit and the rectifier; and

Figure 4 is a portion of a chart record showing the time-break.

In geophysical prospecting, it has been the practice to make use of a plurality of portable seismometers arranged in' a line on or near the surface of the ground and variously spaced from a source of artificially propagated seismic waves.

The seismometers are appropriately connected through amplifiers, if necessary, to each record a trace on a single chart by means of a recording galvanometer having multiple responsive elements. In addition to making a record of the seismic waves arriving at the various seismometers, it is also essential to have on the same chart a very accurate indication of the time of the blaster other means for propagating the seismic waves. Since the blast takes place before any of the seismic waves arrive at the seismometer stations, it saves one element in the recording galvanometer if the time of the blast, called hereafter the time-break, can-be recorded by one of the elements which later records the action of one of the seismometers.

To carry out this dual use of a galvanometer element satisfactorily, it is essential that the re,- cording element be'permitted to return rapidly to its repose condition after receipt of the timebrmk so as to be in readiness fr the recording of the seismic waves. This necessitates some means for treating the time-break signal in such a manner that the recording element of the galvanometer is not unduly disturbed thereby, nor engaged in recording this signal for a prolonged period, but can be quickly damped to substantially zero movement.

Heretofore, it has been customary to obtain a time-break signal as the result of changes in current in the blasting circuit aiorded by the breaking of the bridge wire which detonates the blasting cap, but this has at times been unreliable because of current leakage in the wires leading down into the so-called shot hole. The charge is usually placed some distance beneath Athe surface of the earth in a drilled hole which is lled with Water so the possibility of leakage is greatly increased.

In accordance with the present invention it is proposed to make use of the voltage changes prior to, resulting from, and subsequent to the break in the bridge wire to form the time-break signal. This permits much more accurate recording of the time-break, and permits the use of a vacuum tube rectier between the blasting circuit and the recording equipment which insures complete isolation of the blasting circuit and any filters which may be essential to eliminate the blasting current ripples from the recording equipment. Heretofore it has always been considered highly dangerous to have any lter condensers of substantial capacity associated with the blasting circuit for fear they might not be properly discharged after prior use. This could result in a premature blast by condenser discharge through the bridge wire of the cap. The use of a thermionic tube between the cap and the lters positively precludes such accident since such tubes are essentially one-way valves.

For a complete understanding of the invention, reference should be had first to Figure 1 of the drawing where most of the component parts of the equipment necessary for seismic prospecting are illustrated schematically. In that drawing there is shown at I a conventional blasting cap having the usual bridge wire I I which is electrically heated to ignite the fulminate in the cap and thus detonate the explosive charge in which the cap is embedded. The bridge wire is connected by conductors to the terminals I2 and I3 of the blaster I4 which is a more or less conventional piece of equipment comprising a direct current generator adapted for manual operation by the pushing of a plunger handle carrying a rack operating on a pinion mounted on the armature shaft of the generator. Unlike the usual arrangement where the generator circuit is normally open until the plungery reaches approximately the end of its stroke, when it engages and closes a switch, it is preferred to have the blaster generator c-onnected to the cap at all times throughout the plunger stroke.

In accordance with the present invention the signal initiated by the breaking of the bridge wire, which occurs as a result of the explosion, is a function of voltage changes in the cap-blaster circuit rather than, as in previous methods, a record of current changes. In order to use the current changes, apparatus was required to be connected in series between the blaster and the bridge wire of the cap. The present apparatus is connected in parallel as clearly shown, the conductors I5 and I6 leading from the terminals of the blaster to the terminals I1 and I8 of the so-called shooter's box I9 which is a piece of equipment for use by the operator who does the blasting. It serves to associate` the signal generating energy with a. pair of conductors leading to the station where the time signal and seismic wave signals are recorded. This box I9 also contains equipment permitting direct telephonie communication, over these same conductors, between the operator of the blasting equipment and the operator of the recording equipment'. The output terminals 20 and 2| of the shooters box I 9 are connected to the cable conductors 22, 23 which lead to the recording station.

At the recording station the conductors 22, 23 are connected to the terminals of the primary winding 24 oi the transformer 25, the terminals of the secondary 26 of which are connected to a pair of the terminals 21 of the recording galvanometer 28. This galvanometer is equipped with a plurality of recording elements all of which are adapted to make a record on a single sheet or chart which is moved by clockwork into the paths of shadowscast by the moving elements.

As previously stated, each of the galvanometer elements is adapted for connection to a seismometer whereby a record may be made of the Waves received by that seismometer at its particular station as a result of the artificially created seismic disturbance at the blast. One such seismometer is shown at 3l) connected through an appropriate thermionic amplifier 3|, of known type, to the same terminals 21 of the recording galvanometer to which the secondary of the transformer 25 is connected. This permits the use of one moving element in the galvanometer for recording both the time-break and the waves received by seismometer 30 spaced a distance from the "shot point.

At the receiving or recording end of the conductors 22, 23 provision is made for connecting a telephone instrument 32 which bridges the condenser 33 as shown. The battery 34 provides microphone current for the telephone without interfering with the passage of the high frequency signals from the blaster which also pass through the condenser 33 without diiiiculty. If desired, a double-pole, double-throw switch may be substituted for the condenser 33 to selectively connect the transformer 25 or the telephone equipment to the line. 'I'he arrangement of Asuch a switch will be obvious.

The circuits of the equipment in the shooters box I9 are shown in detail in Figure 2 where similar reference characters have been used for the respective terminals. The terminals I1 and I8 are shunted by a potentiometer 35, of several hundred ohms resistance, having an adjustable tap 36. The output from this potentiometer passes through a pair of high resistances 31 and 38 on the order of 100,000 ohms and is thence connected to the input circuit of the thermionic tube 40 by being connected respectively to the grid 4I and cathode 42 thereof in the usual manner. This cathode is heated by a battery 43, the circuit of which is closed at either of the contacts T or Blc of switch 39 or opened at the contact marked zero.

The anode 44 of the tube 40 is energized by batteryv 45, the return 46 of which leads to Bk contact 41 of a switch.48. When this switch ish engaging the Bk contact the return conductor from the anode and battery passes through the primary 50 of the transformer 5I, thence through milliammeter 52, and nally to the cathode at switch 39.

The switch 48 is shown resting on the open or assente zero contact at the center. I'here is also a. third contact 54 marked T. From contact 54 leads a conductor having interposed therein a conventional jack 55. which when 'the switch 48 is set on contact 54 is in shunt to the primary 50, with the tube anode circuit open. This jack is adapted to receive a plug from the head telephones 56 shown in Figure 1.

The secondary 58 of transformer 5| has one terminal connected to output terminal 2| of the shooters box I9, here shown as one spring of a jack. The second terminal leads to contact Bic 59 which when engaged by the switch 60 leads to the second terminal 20 of the shooters box or the second element of the jack. 'I'his jack is'adapted to receive a, plug connected to the conductors 22 and 23 of the main cable. The switch 50 may alsobe set at a zero contact, as shown, or at a third or T contact 6|. When engaged with 6| a second jack 62 is arranged in series between the line jack and the secondary 58, which second jack is adapted to receive the plug of the microphone 63. y

The telephone circuits will be obvious from the drawing. With an appropriate microphone plugged in jack 62 and head-phones in jack 55 and all three switches, which in the actual construction are ganged together for operation by the single knob 64 (Figure l), set to the T contacts, the secondary of transformer is connected through the line in series with the primary of the transformer 25 having interposed both microphones and the battery 34. The headphones 56 are available for use through the transformer 5|. The tube has its output open-circuited' by switch 48, but the filament is maintained in a heated condition by the lament switch even though positioned on T, so that the circuit is ready to be immediately switched over for the recording of the time-break.

When the apparatus is not in use, all switches are set simultaneously to the zero position, opening the circuits to all batteries and insuring no discharge of the same.

When the operators are in communication by the telephones just described and the blasting operator is advised of the readiness of the recording equipment for the blast, the switches are set to the Bk position, the telephones being thereby disconnected. The milliammeter 52 is examined to insure that current is flowing to the plate of the tube, and the potentiometer switch 36 is adjusted to a value which in the judgment of the operator will give the proper sized time-break signal on the chart. When all is in readiness the blaster is operated and a voltage, increasing slowly as the blaster attains speed, is applied to the terminals l1, I8 which renders the grid 4| more positive and produces a slow increase in the flow of plate current to the tube as shown in Figure 4. The previous steady plate current flow through the primary of transformer 5| has not caused any current flow in the line, and this slow increase in plate current ow produces only a small impulse over the line to the recording galvanometer. This impulse would be full of high frequency ripples from the commutator on the blaster if it were not for the rectifying action of the tube 40 together with the filter action of the combined inductance of the primary winding 50 and shunt condenser 66 which lterout the high frequency in the signal and cause only a smooth slowly changing current to travel over the line.

In the matter of a few hundredths of a second after a current is applied by the blaster the cap is exploded by the heated bridge wire and the explosion disrupts this wire relieving@ a. large portion of the load on the blasting machine so that the voltage thereof increases. This produces a wave impulse over the line for recording by the galvanometer as shown. The cessation of voltage when the blaster stops is rapid and( smooth as shown by the rapidly falling line in Figure 4, bringing the string of the galvan'ometer to quiescence and ready for further use.

The primary function of the tube 40 is to act in a rectifying capacity to thereby provide an envelope of the total signal generated during the operation of the blaster. This envelope contains all of the pulsations, now rectified, present in the signal, but these pulsations where of high frequency are removed by the' condenser 66, which is of large size, and the inductance of the winding 50, so that the final envelope is a relatively smooth curve with a sharp rise indicating the occurrence of the bridge wire breaking and this is the so-called time-break so essential on the record. This rectifying and filtering action is of great importance and permits the use of the same recording element in the galvanometer for a seismometer recording and the time-break record since the filtering action and the smooth voltage variation applied to the element do not set it into violent oscillations which cannot be damped out in time to have the element substantially quiet when the seismic signals from 30 arrive for recording. They will naturally arrive some hundredths of a second subsequent to the time-break and in this relatively short time the element must be quieted. It cannot be properly quieted unless the signal from the timebreak is of the benign character just described. The apparatus just described is so satisfactory in actual operation that, in addition to one of the regular, remote seismometers, a second one 10 can be'placed on the same trace. This is the socaliedtophole seismometerset just at the top of the shot hole and used to record the time of travel of the seismic waves vertically to the surface of the earth from the deep blast. Since this is the rst seismometer to be actuated it will be clear that the time-break signal must be connected to the`blasting cap without having y been previously discharged. Premature explosions have so resulted and such circuits have hence been considered undesirable. Since the rectifyi'ng tube is an entirely one-way valve no' discharge of the condenser 66 can cause any current fiow in the blaster or cap circuit, but to make certain that, even in the event of shortcircuiting of the elements in the tubes, adequate current cannot ow to discharge the cap, the resistors 3l' and 38 are arranged in the circuit as shown. Their resistance is too great to permit the ow of sufiicient current to even heat the bridge wire moderately.

A third function of the tube is to act in an amplifying capacity so that the relatively small changes in voltage occurring in the blasting circuit can be amplied to a sumcient value for transmission over the relatively long conductor line to provide sumcient energy for operation of the galvanometer element. A tube having the proper amplication factor can readily be selected for this purpose since the output made use of is primarily the voltage, a small amount or current being adequate for the purpose of this invention.

It will be obvious that the system just described is as satisfactory for radio communication as for line wire. The changes necessary for conversion are so simple as not to need illustration.

The tube control voltage may of course be obtained from the blasting circuit by any desired manner of coupling other than the direct resistance method shown. For example a transformer of any desirable characteristics could be used.

An arrangement which has been found satisfactory after considerable ileld use is that shown in Figure 3. This takes the place, in Figure 2, of the potentiometer 35 and the resistances 31 and 38'. In this arrangement the blasting machine is connected to the terminals 'l0 and 1l, the current passing through the iron core inductance 12 from conductor 10 to terminal 13 for connection to one side of the cap. The other side of the cap, connected to terminal 14, has a connection to conductor 7|. A resistance 'I5 of suitable value shunts the cap leads as clearly shown, and in parallel with this resistance is the primary 16 of a transformer 71, the secondary 18 of which is connected to the input of the thermionic tube M, the remainder of Whose circuit is identical with that of Figure 2.

In this arrangement two items not illustrated in Figure 2 are available for better control to overcome certain trying conditions which occur in the field. 'I'he choke 12 in the cap circuit serves to eliminate some of the high-frequency ripple from the commutator of the blaster, which for some reason certain field conditions seem to accentuate. Furthermore, this inductance tends to raise the voltage across the cap terminals with much greater rapidity when the cap breaks than would the voltage rise if the choke were not in the circuit.

The transformer has one main functionthat of keeping the direct current component of [the blasting machine voltage out of the grid circuit of the tube thereby preventing the tube from becoming blocked by this component.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. Apparatus for recording the time-break of a blast for seismic surveying initiated by the heating of a cap bridge Wire comprising, in combination, a thermionic tube, means connecting the tube with its input in shunt to the said bridge Wire, a high frequency lter in the output of said tube, and a. recording galvanometer associated with said output.

2. Apparatus for recording the time-break of a blast for seismic surveying comprising, in combination, the bridge wire of a blasting cap, a source of current connected to heat the same, a thermionic tube having its input shunting said Wi-re, a recording galvanometer associated with the output of said tube. a source of current and a high capacity in the output circuit of said tube and separated by the tube from the said bridge wire.

3. Apparatus for recording the time-break of a blast for seismic surveying comprising the combination with the bridge Wire of a blasting cap and a blasting generator connected to heat the same, of a rectiier having its input shunting said Wire, a recording galvanometer, a iilter and circults associating the output of said rectiiier, the lter and the g-alvanometer so constructed and operated that the voltagechanges taking place iu the bridge Wire blaster circuit are rectified, lltered and applied to the galvanometer for recording as a relative smooth curve having a single sharp deflection indicating the time-break.

d. Apparatus for recording the time-break of a blast for seismic surveying comprising the combination with a bridge wire of a blasting cap, a blasting generator connected to heat the same, and a recording galvanometer, of a circuit associating said wire, blaster and galvanometer so arranged that voltage changes in the blasterbridge wire combination are recorded in respect to time. a filter in said circuit including a capacity, `and means between the capacity and Wire to prevent discharge of the capacity into the wire.

5. Apparatus for recording the time-break of a blast for seismic surveying comprising the combination with fthe bridge wire of a blasting cap, a. blasting generator connected to heat the same, and a recording galvanometer, of a circuit associating said blaster, wire and galvanometer whereby voltage changes in the blaster-bridge wire combination are recorded in respect to time, a filter in said circuit including a capacity and a thermionic tube between the capacity and Wire, said tube being connected to prevent current flow from the capacity to the wire.

6. Apparatus for recording the time-break of a blast for seismic surveying comprising the combination with the bridge Wire of a blasting cap, a blasting generator connected to heat the same, and a recording galvanometer, of a circuit associating said blaster, wire and galvanometer whereby voltage changes in the blaster-bridge Wire combination are recorded in respect to time, a fllter in said circuit including a capacity, a thermioni-c tube between the capacity and Wire, said tube being connected to prevent current flow from the capacity to the Wire, and means to prevent high rate discharge of the condenser through said wire in the event of short-circuited tube elements.

7. The combination with an electric seismometer at top hole position, of a blasting circuit, said circuit comprising a manually operated direct current type magneto-electric machine and the bridge Wire of a blasting cap connected in series, a recording galvanometer connected to said seismometer, means connecting said blasting circuit to said galvanometer so constructed and arranged that signal voltages induced in the blasting circuit at the time of break of the bridge wire may be so modied that they are transmitted to the galvanometer and ful-ly recorded before the seismometer is energized by the blast from said cap, said means including a rectifier element sensitive to only voltage changes in the blasting circuit, and means between said element and galvanometer to eliminate only high frequency pulsations from said signal.

8. The combination with an electric seismometer, of a blasting circuit, said circuit comprising a snurce of d-irect current and the bridge wire of a blasting cap connected in series, a recording galvanometer connected to said seismometer, means associating said blasting circuit and said galvanometer so constructed and arranged that signal voltages induced in .the blasting circuit at the time of break of the bridge wire may be so modied that they are transmitted to the galvanometer, said means including a rectifier for the signal voltages and a high frequency lter for the rectified signal.

9. The combination with an electric seismometer, of a blasting circuit, said circuit comprising a source of direct current and the bridge wire of a blasting cap connected in series, a recording galvanometer connected to said seismometer, means associ-atingl said blasting circuit and said galvanometer so constructed and arranged that lsignal voltages induced in the blasting circuit at the time of break of the bridge wire may be so modified that they are transmitted to the galvanometer, said means including a 'thermionic amplier for the signal voltages, a lter having a high capacity in ther output of said amplifier,

and means to limit the rate of discharge `of said capacity through said wire in the event of ashort circuit in said amplifier whereby it will not be heated to cap exploding temperature.

WILLIAM HARRY MAY/NE. 

